1. Field of the Invention
The present invention relates to a memory device configured by electrically rewritable non-volatile memory cells, and in particular, to a non-volatile memory device using variable resistive elements as memory elements to store the resistance values as data, and a method for writing data thereto.
2. Description of the Related Art
Recently, ReRAM (Resistive Random Access Memory) has gained increasing attention as a non-volatile memory device that stores resistance value information of electrically rewritable variable resistive elements in a non-volatile manner, which is a technology currently studied in various research laboratories.
The ReRAM has variable resistive elements as its storage elements that are configured by layers of an electrode/a metal oxide/an electrode. The variable resistive elements are known to have two types of operational modes. One operational mode involves switching between a high-resistance state and a low-resistance state by changing polarity of the applied voltage, which is referred to as “bipolar type”. The other involves switching between a high-resistance state and a low-resistance state by controlling values and application time of voltage without changing polarity of the applied voltage, which is referred to as “unipolar type” (or “non-polar type”).
The unipolar type is preferable to achieve high-density memory cell arrays. This is because the unipolar type enables cell arrays to be configured by superposing a variable resistive element and a rectifying element such as a diode on each cross point of a bit line and a word line without any transistors.
When cross-point type cell arrays are configured without any rectifying elements, a write disturbance occurs in writing at non-selected cells due to crosstalk. In order to mitigate such effects, such a scheme is disclosed in Patent Document 1 where a write-compensation voltage is applied to the non-selected memory cells after the application of a write voltage.
On the other hand, to prevent such crosstalk in cross-point type cell arrays, a rectifying element such as a diode may be superposed on a variable resistive element (see, for example, Patent Document 2).
For example, a homopolar voltage may be used for writing and erasing and that shorter pulses may be used for writing while longer pulses used for erasing in cross-point type cell arrays, which is disclosed in, e.g., non-Patent Document 1.
[Patent Document 1] Japanese Patent Laid-Open No. 2006-344349
[Patent Document 2] U.S. Pat. No. 6,831,854
[Non-Patent Document 1] Y. Hosoi et al, “High Speed Unipolar Switching Resistance RAM (RRAM) Technology” IEEE international Electron DEVICES meeting 2006 Technical DIGEST p. 793-796